It was discussed. SimpleX is extremely complicated. It's barely maintained. For example, for JS there is https://github.com/simplex-chat/simplexmq-js - but as per SimpleX founder words, it's abandoned and represents less than 5% of required code.

For comparison, full NIP-44 js implementation fits in just ~100 lines.

SimpleX is not a solution for nostr. Integrating simplex server functionality into relays is also extremely complicated. If you think you can do this - submit a proposal, and write some code, but it doesn't seem to be worth it.